Head-mounted display (HMD) technologies have been applied to a wide range of scientific and engineering domains. Examples of applications include flight simulation, scientific visualization, medicine, engineering design, education and training, wearable computing, and entertainment systems. In the domain of augmented reality, HMDs are one of the enabling technologies for merging virtual views with physical scenes, which may enable a physician to see a 3D rendering of the anatomical structures or CT images of a patient superimposed onto the patient's anatomy, such as the abdomen, for example. In the domain of wearable computing, an HMD creates a mobile display solution that offers much more attractive image quality and screen size than other popular mobile platforms such as smart phones and PDAs. In the foreseeable future, such mobile displays may appear as elegant as a pair of sunglasses and may become an integral part of many people's daily activities to retrieve information and connect with people instantly.
In parallel with HMD technologies, various eyetracking technologies have been developed and applied to several disciplines including vision research, human computer interfaces, tele-operation environments, and visual communication. The benefits of eyetracking for multi-modal human-computer interfaces and the technical benefits of data compression have been well-recognized and studied. For instance, multi-resolution gaze-contingent display and image processing schemes have been proposed to effectively save data transmission bandwidth in communication, and improve rendering speed of 3D scenes using foveated level-of-detail management methods, and to achieve wide FOV high-resolution display and imaging systems.
The concept of creating an integrated eyetracked HMD (ET-HMD) system has been explored in various levels. An ET-HMD is able to display monocular or stereoscopic virtual images as a classical HMD does, while additionally tracking the gaze direction of the user. A fully-integrated ET-HMD offers multi-fold benefits, not only to fundamental scientific research but also to emerging applications of such technology. For instance, many research efforts are concerned about how human users perceive and organize spatial information, interact with such information, and navigate within 3D virtual spaces. Eyetracking capability in HMDs adds a very valuable tool and objective metric for scientists to quantitatively assess user interaction with 3D environments and investigate the effectiveness of various 3D visualization technologies for various specific tasks including training, education, and augmented cognition tasks. From the technology point of view, eyetracking capability integrated with HMD systems can be utilized to improve size and depth perception accuracy in stereoscopic displays. Eyetracking capability may help to create solutions to the FOV-resolution tradeoff through a fovea-contingent display scheme and to the accommodation-convergence contradiction by using vari-focal plane display methodology. From the application point of view, an ET-HMD offers unique opportunities for novel interactive interfaces for people with proprioceptive disabilities where eye gaze instead of hands or feet can be used as a method of interaction and communication.
Despite significant advancements and commercial availability of stand-alone HMD and eyetracking technologies, integrating these two stand-alone technologies imposes significant challenges in creating a compact, portable, accurate and robust system. Although several pioneering efforts were made to develop ET-HMD technologies and to optimize these two technologies in a systematic approach, none of the existing technological solutions offers a truly portable, lightweight, and robust system that conforms to the form factor of an eyeglass-style display. For many demanding applications, lightweight and compactness are critical. For instance, to support Amyotrophic Lateral Sclerosis (ALS) patient communication, the integrated system has to be lightweight so that the patients are able to bear the weight with their significantly weakened muscles and very limited mobility.
Over the past decades, many different optical design approaches have been applied to HMD designs to improve the system performance. These methods include applying catadioptric technique, introducing new elements such as aspherical surfaces, using holographic and diffractive optical components, exploring new design principles such as using projection optics to replace an eyepiece or microscope type lens system in a conventional HMD design, and introducing tilt and decenter or even freeform surfaces. Few of these optical design methods are capable of creating a wide field-of-view, compact, and lightweight HMD that is nonintrusive and can be considered as being eyeglass-style near-eye displays. Integrating eyetracking capability to these technologies is very challenging and adds significant weight, volume, and complexity.
Adding eyetracking capability to HMDs started as early as the high resolution inset displays by CAE Corporation. This pioneering work was not intended for mobile compact ET-HMD systems. Also, others used a mechanical driving device to move a high resolution inset in a bench-prototype stereoscopic display. ISCAN Corporation worked to integrate an ISCAN eyetracker into a V8-HMD from Virtual Research Corporation to study software-based fovea-contingent display scheme. This method of integrating commercially available HMDs and eye-trackers is referred to as the functionality integration approach, in which two separate instruments are brought together at a later stage of utilization. Though the functionality integration approach has the advantage of being a simple solution with low development cost, it generally does not take advantage of low-level optimization and lacks the attributes of compactness, accuracy, and robustness.
In contrast to the functionality integration approach, a systematic approach, where the system is conceived and optimized as one single instrument from a fundamental design perspective, has many advantages in creating a fully integrated ET-HMD instrument. The significant benefits of the systematic approach include the ability to explore the design constraints and requirements for both the display and eyetracker units, conceive new solutions, and optimize the designs for a compact and robust system. Pioneering efforts have been made to explore the possibility of a complete integration with low-level optimization. Following these earlier efforts, Hua and Rolland collaboratively pursued a fully integrated design approach, developed robust eyetracking methods and algorithms for an ET-HMD system, and designed an optical see-through ET-HMD optical system based on the concept of head-mounted projection displays. FIG. 1 shows the first-order layout of the ET-HMD optical system, in which the optical system was simplified with ideal lens modules to emphasize the concept and the scale. (Curatu, C., Hong Hua, and J. P. Rolland, “Projection-based head-mounted display with eye-tracking capabilities,” Proceedings of the SPIE International Society for Optical Engineering, Vol. 5875, San Diego, USA, August 2005. Curatu, C., J. P. Rolland, and Hong Hua, “Dual purpose lens for an eye-tracked projection head-mounted display,” Proceedings of International Optical Design Conference, Vancouver, Canada, June 2006.). The design took a full integration approach and combined most of the optical paths for the display and eyetracking subsystems. The same projection optics was shared for both display and eye imaging functions. The main limitation of this design, however, was that the overall volume of the integrated ET-HMD system, although significantly improved over others, was still bulky and heavy.
The key challenges of creating a truly portable, lightweight, compact ET-HMD solution lies in addressing two cornerstone issues: (1) an optical method that enables the design of an HMD system with an elegant form factor as compelling as a pair of sunglasses, which has been a persistent dream for both technology and application developers; and (2) an optical method that allows the integration of the eyetracking capability without adding significant weight and volume to the system.